I get it, I know nothing of the project. More so just pointing out IMO this problem often starts before development. It's often a false notion to think the same content isn't relevant across multiple devices. If you agree tell your client they are wrong

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I know it may seem "pages need to be lighter for speed" for mobile devices but is loading one DOM, detecting viewport with JS, and then redirecting to another page really faster?
Different interaction sure, that can be handled by detecting .touch vs .no-touch or of course viewport size but let's not forget, that can change at any time.
You can do a lot now but once position:sticky lands in CSS3 I think we will see a lot more creative uses of adapting even large amounts of content to mobile without hiding them.